A conventional device for monitoring the flow of fluid through a conduit employs a blade which extends into the conduit and is subjected to the force of the moving fluid. In one form of prior art device, the actual movement of the blade is limited. An example of such a switch is that described in U.S. Pat. No. 3,729,604 of Groff et al. The larger the conduit, the larger the blade that is employed. Since the blade moves only slightly under the force of the fluid, it tends to flex and, eventually, break. Another disadvantage of such a construction is that sediment contained within the fluid tends to build up due to eddies and turbulence. Another problem arises from the need to seal the switching cavity from the fluid in the conduit. This is commonly accomplished by means of an elastomeric seal or by a metallic bellows. With either approach the sealing device tends to wear out or deteriorate.
Another approach is illustrated in U.S. Pat. No. 3,749,864 of Tice. This patent discloses a construction wherein the blade is movable through a 180.degree. arc. However, movement of the blade is used to cam a spring-loaded member which lifts a magnet into the vicinity of a reed switch. The end result is a substantial amount of friction during the act of camming which reduces the sensitivity of the device. Furthermore, the presence of sharp corners and open cavities tends to induce turbulence and increases sedimentation rates.
The sealing problem is common to all the prior art devices, especially when the fluid being monitored is at high temperatures or pressure or is toxic or corrosive. Accordingly, the objects of the inention are to provide a fluid flow responsive switch which retains a full range of motion of the blade; is usable with a wide variety of monitored fluids; has improved sensitivity to flow variation; minimizes sediment buildup; and has improved sealing qualities intermediate the monitored fluid and the ambient atmosphere.